Triple Polarized Patch Antenna

ABSTRACT

An antenna arrangement for a Multiple Input Multiple Output (MIMO) radio system, the antenna arrangement transmitting and receiving in three essentially uncorrelated polarizations. The arrangement includes first and second patches, and four feeding points for feeding the first patch. In one mode of operation, the feeding points are fed in phase with each other, resulting in a first constant E-field in a slot between the edges of the patches. In a second operating mode, the first and second feeding points are fed 180 degrees out of phase with each other, resulting in a second E-field in the slot having a first sinusoidal variation. In a third operating mode, the third and fourth feeding points are fed 180 degrees out of phase with each other, resulting in a third E-field in the slot having a second sinusoidal variation uncorrelated with the first sinusoidal variation.

TECHNICAL FIELD

The present invention relates to an antenna arrangement comprising afirst and a second patch, each patch being made in a conducting materialand having a first and a second main surface, which patches are placedone above the other with the first patch at the top, such that all ofsaid main surfaces are essentially parallel to each other, in whichantenna arrangement the first patch has a first edge and the secondpatch has a second edge, where furthermore the antenna arrangementcomprises a feeding arrangement, which feeding arrangement comprises afirst, second, third and fourth feeding point, said feeding points beingarranged for feeding the second patch, in transmission as well as inreception, each positioned at a distance from a first imagined linepassing the patches essentially perpendicular to the respective firstand second main surfaces, where a second and third imagined line passesperpendicular to, and intersects, the first line, and where the secondline also intersects the first and second feeding points, and where thethird line also intersects the third and fourth feeding points, thesecond and third line presenting an angle α between each other, theangle α being essentially 90°, such that the clockwise order of thesucceeding feeding points is the first, the third, the second, and thefourth.

BACKGROUND ART

The demand for wireless communication systems has grown steadily, and isstill growing, and a number of technological advancement steps have beentaken during this growth. In order to acquire increased system capacityfor wireless systems by employing uncorrelated propagation paths, MIMO(Multiple Input Multiple Output) systems have been considered toconstitute a preferred technology for improving the capacity. MIMOemploys a number of separate independent signal paths, for example bymeans of several transmitting and receiving antennas. The desired resultis to have a number of uncorrelated antenna ports for receiving as wellas transmitting.

For MIMO it is desired to estimate the channel and continuously updatethis estimation. This updating may be performed by means of continuouslytransmitting so-called pilot signals in a previously known manner. Theestimation of the channel results in a channel matrix. If a number oftransmitting antennas Tx transmit signals, constituting a transmittedsignal vector, towards a number of receiving antennas Rx, all Tx signalsare summated in each one of the Rx antennas, and by means of linearcombination, a received signal vector is formed. By multiplying thereceived signal vector with the inverted channel matrix, the channel iscompensated for and the original information is acquired, i.e. if theexact channel matrix is known, it is possible to acquire the exacttransmitted signal vector. The channel matrix thus acts as a couplingbetween the antenna ports of the Tx and Rx antennas, respectively. Thesematrixes are of the size M×N, where M is the number of inputs (antennaports) of the Tx antenna and N is the number of outputs (antenna ports)of the Rx antenna. This is previously known for the skilled person inthe MIMO system field.

In order for a MIMO system to function efficiently, uncorrelated, or atleast essentially uncorrelated, transmitted signals are required. Themeaning of the term “uncorrelated signals” in this context is that theradiation patterns are essentially orthogonal. This is made possible forone antenna if that antenna is made for receiving and transmitting in atleast two orthogonal polarizations. If more than two orthogonalpolarizations are to be utilized for one antenna, it is necessary thatit is used in a so-called rich scattering environment having a pluralityof independent propagation paths, since it otherwise is not possible tohave benefit from more than two orthogonal polarizations. A richscattering environment is considered to occur when many electromagneticwaves coincide at a single point in space. Therefore, in a richscattering environment, more than two orthogonal polarizations can beutilized since the plurality of independent propagation paths enablesall the degrees of freedom of the antenna to be utilized.

Antennas for MIMO systems may utilize spatial separation, i.e. physicalseparation, in order to achieve low correlation between the receivedsignals at the antenna ports. This, however, results in big arrays thatare unsuitable for e.g. hand-held terminals. One other way to achieveuncorrelated signals is by means of polarization separation, i.e.generally sending and receiving signals with orthogonal polarizations.

It has then been suggested to use three orthogonal dipoles for a MIMOantenna with three ports, but such an antenna is complicated tomanufacture and requires a lot of space when used at higher frequencies,such as those used for the MIMO system (about 2 GHz). Up to six portshave been conceived, as disclosed in the published application US2002/0190908, but the crossed dipole and the accompanying loop elementis still a complicated structure that is difficult to accomplish forhigher frequencies to a reasonable cost.

The objective problem that is solved by the present invention is toprovide an antenna arrangement suitable for a MIMO system, which antennaarrangement is capable of sending and receiving in three essentiallyuncorrelated polarizations. The antenna arrangement should further bemade in a thin structure to a low cost, and still be suitable for higherfrequencies, such as those used in the MIMO system.

DISCLOSURE OF THE INVENTION

This objective problem is solved by means of an antenna arrangementaccording to the introduction, which antenna arrangement further ischaracterized in that, in a first mode of operation, each one of thefeeding points are fed essentially in phase with each other, resultingin a first constant E-field being obtained in a slot created between thefirst and second edges, which first E-field further is directed betweensaid edges, and, in a second mode of operation, the first and the secondfeeding points being fed essentially 180° out of phase with each other,resulting in a second E-field in the slot, which second E-field furtheris directed between said edges and has a sinusoidal variation along theslot, and, in a third mode of operation, the third and the fourthfeeding points being fed essentially 180° out of phase with each other,resulting in a third E-field in the slot, which third E-field further isdirected between said edges, and has a sinusoidal variation along theslot.

Preferred embodiments are disclosed in the dependent claims.

Several advantages are achieved by means of the present invention, forexample:

-   -   A low-cost triple polarized antenna arrangement is obtained.    -   A triple polarized antenna made in planar technique is made        possible, avoiding space consuming antenna arrangements.    -   A triple polarized antenna which is easy to manufacture is        obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described more in detail withreference to the appended drawings, where

FIG. 1 a shows a schematic simplified perspective view of a firstembodiment of the antenna arrangement according to the invention;

FIG. 1 b shows a schematic side view of a first embodiment of theantenna arrangement according to the invention;

FIG. 1 c shows a schematic top view of a first embodiment of the antennaarrangement according to the invention;

FIG. 2 a shows a schematic simplified side view of the fielddistribution at the patches of the antenna arrangement according to theinvention at a first mode of operation;

FIG. 2 b shows a schematic simplified side view of the fielddistribution at the patches of the antenna arrangement according to theinvention at a second mode of operation; and

FIG. 2 c shows a schematic simplified side view of the fielddistribution at the patches of the antenna arrangement according to theinvention at a third mode of operation.

PREFERRED EMBODIMENTS

According to the present invention, a so-called triple-mode antennaarrangement is provided. The triple-mode antenna arrangement is designedfor transmitting three essentially orthogonal radiation patterns.

As shown in FIG. 1 a-b, illustrating a first embodiment of the presentinvention, a triple-mode antenna arrangement 1 comprises a first 2 andsecond 3 patch. Each patch 2, 3, is relatively thin, having a centrepoint, and a first 4, 5 and a second 6, 7 main surface, which first andsecond main surfaces 4, 5; 6, 7 are essentially parallel to each other.The patches 2, 3 are made in a conducting material, such as copper. Thepatches 2, 3 are preferably round in shape and placed one above theother with the first patch 2 at the top. The patches 2, 3 also havecorresponding first and second edges 8, 9.

The triple-mode mode antenna arrangement 1 further comprises a first 10,second 11, third 12, and fourth 13 coaxial feed line, having a first 14,second 15, third 16, and fourth 17 centre conductor, respectively.

The first 14, second 15, third 16, and fourth 17 centre conductor eachmakes electrical contact with the first patch 2 in its outer area, thereconstituting a first 18, second 19, third 20 and fourth 21 feedingpoint. Also with reference to FIG. 1 c, the first 18, second 19, third20 and fourth 21 feeding points are positioned at an appropriatedistance d from a first imagined line 22 passing through the centre ofthe patches 2, 3, essentially perpendicular to the main planes 4, 5; 6,7. The distance d is preferably essentially the same for the first 18,second 19, third 20 and fourth 21 feeding points.

A second 23 and third 24 imagined line passes perpendicular to the firstimagined line 24 and each intersect the first 18, second 19, third 20and fourth 21 feeding points, presenting an angle α between each other.This is a way to define the angle α between feeding points, the angle αis essentially 90°. The defining of an angle between feeding points inthe above manner is referred to as an angular displacement further inthe text. The imagined lines 22, 23, 24 are inserted for explanatoryreasons only, and are not part of the real device 1.

There is thus an angular displacement of essentially 90° between thesucceeding feeding points 18, 20, 19, 21 all the way around thecircumference of a circle with the radius d. The succeeding feedingpoints 19, 21, 18, 20 are then positioned in such a way that the first18 and second 19 feeding points are opposite each other with the firstimagined line 22 positioned between them, and the third 20 and fourth 21feeding points are opposite each other with the first imagined line 22positioned between them, the clockwise order of the succeeding feedingpoints being the first 18, the third 20, the second 19, and the fourth21.

The feeding coaxial lines 10, 11, 12, 13 with their centre conductors14, 15, 16, 17 are part of a feeding arrangement.

The first 14, second 15, third 16 and fourth 17 centre conductors makeno electrical contact with the first patch 2, and mainly extendperpendicular to the main surfaces 4, 5; 6, 7 of the patches 2, 3. Thefirst 10, second 11, third 12 and fourth 13 coaxial feed lines passesthrough the outer area of the second patch 3 by means of holes 25, 26,27, 28 made into this, through which the coaxial feed lines 10, 11, 12,13 may run.

The electrical contact between the first patch 2 and the belongingcentre conductors 14, 15, 16, 17 at the corresponding feeding points 18,19, 20, 21 is for example obtained by means of soldering.

With reference to FIG. 1 a, the feeding arrangement further comprises afirst 29 and a second 30 four-port 90° 3 dB hybrid junction and a first31 and second 32 90° phase-shifter. Each four-port 90° 3 dB hybridjunction 29, 30 has four terminals, A, B, Σ and Δ. If the Δ terminal isconnected to its characteristic impedance, an input signal at the Σterminal is divided into two signals at the A and B terminal, eachsignal having the same amplitude with the phase at the A terminalshifted −90°. If, on the other hand, the Σ terminal is connected to itscharacteristic impedance, an input signal at the Δ terminal is dividedinto two signals at the A and B terminal, each signal having the sameamplitude with the phase at the A terminal shifted +90°. The function isreciprocal. For reasons of clarity, the first 29 and a second 30four-port 90° 3 dB hybrid junction and the first 31 and second 32 90°phase-shifter are only shown in FIG. 1 a.

The first four-port 90° 3 dB hybrid junction 29 comprises a differenceterminal Δ₁, a sum terminal Σ₁ and two signal terminals A₁ and B₁.Further, the second four-port 90° 3 dB hybrid junction 30 comprises adifference terminal Δ₂, a sum terminal Σ₂ and two signal terminals A₂and B₂. The sum terminals Σ₁ and Σ₂ are connected to a common sum signalport 33 at a sum connection point 33′. The difference terminals Δ₁, Δ₂are connected to a first 34 and second 35 difference port, respectively.

Further, as shown schematically in FIG. 1 a, the coaxial feed lines 10,11, 12, 13 of the feeding network leading from the first 29 and second30 90° 3 dB hybrid junctions, which coaxial feed lines 10, 11, 12, 13are of equal lengths excluding the first 31 and second 32 phaseshifters, feed the first patch 2 at the four feeding points 18, 19, 20,21. The signal terminal A₁ is connected to the first feeding point 18 bymeans of the first coaxial feed line 10, via the first phase shifter 31,and the signal terminal A₂ is connected to the third feeding point 20 bymeans of the third coaxial feed line 12 via the second phase shifter 32.Further, the signal terminal B₁ is connected to the second feeding point19 by means of the second coaxial feed line 11 and the signal terminalB₂ is connected to the fourth feeding point 21 by means of the fourthcoaxial feed line 13.

By means of the feeding arrangement, the patches 2, 3 may be excited inthree different ways, in a first, second and third mode of operation,enabling three orthogonal radiation patterns to be transmitted.

At all modes of operation described below, the second patch 3 then actsas a ground plane for the first patch 2.

For the first mode of operation, the sum signal port 33 is fed with asignal to the sum connection point 33′, which signal first is dividedequally, and further fed in the same phase to the respective sum port Σ₁and Σ₂ of the 90° 3 dB hybrid junctions 29, 30. The 90° 3 dB hybridjunctions 29, 30 then divide the respective input signal in equalportions, which are output at the respective signal terminal A₁ and B₁and A₂ and B₂, respectively, with the signals at the terminals A₁ and A₂shifted −90°. The signals from A₁ and A₂ are fed through the respective90° phase shifter 31, 32, which may be a discrete component or anadjustment of the coaxial feed line length corresponding to 90°. Thismeans that after the respective phase shifter 31, 32, the signal fromthe terminals A₁ and A₂ are shifted +90°, resulting in a total phaseshift of −90°+90°=0°. All four feeding points 18, 19, 20, 21 are thusfed in phase.

Also with reference to FIG. 2 a, which for reasons of clarity shows thepatches without the feeding arrangement, as the outputs from the signalterminals B₁ and B₂ are not phase shifted at all, this results in aconstant magnetic current loop 36 running in a circumferential slot 37created between the edges 8, 9 of the first and second 3 patch,respectively.

This magnetic current 36 corresponds to a first E-field 38, all aroundthe circumference of the first 2 and second 3 patch, which first E-field31 is constant and directed essentially perpendicular to the mainsurfaces 4, 5; 6, 7 of the first 2 and second 3 patch in the slot 37. InFIG. 2 a, this is shown with a number of arrows.

For the second mode of operation, with reference to FIG. 1 a, a signalis fed to the first difference terminal Δ₁ of the first 90° 3 dB hybridjunction 29 via the first difference port 34. The first 90° 3 dB hybridjunction 29 then divides the input signal in equal portions, which areoutput at the respective signal terminal A₁ and B₁, with the signal atthe terminal A₁ shifted +90°. The signal from A₁ is then fed through thefirst 90° phase shifter 31. This means that after the first phaseshifter 31, the signal from the terminal A₁ is shifted +90°, resultingin a total phase shift of 90°+90°=180°.

Also with reference to FIG. 2 b, as the outputs from the signal terminalB₁ is not phase shifted at all, this results in the first patch 2 beingfed with equal amplitude, but with a phase difference of 180° at theopposite first 18 and second 19 feeding points.

This in turn results in a second E-field 39 directed essentiallyperpendicular to the main surfaces 4, 5: 6, 7 of the first 2 and second3 patch in the circumferential slot 37 created between the edges 8, 9 ofthe first 2 and second 3 patch, respectively, having a sinusoidalvariation all around the circumference of the first 2 and second 3patch. The E-field 39 is shown in FIG. 2 b as a number of arrows havinga length that corresponds to the strength of the E-field, where thearrows indicate an instantaneous E-field distribution as it variesharmonically over time.

With reference to FIG. 1 a, the third mode of operation corresponds tothe second mode of operation, but here a signal is fed to the seconddifference terminal Δ₂ of the second 90° 3 dB hybrid junction 30 via thesecond difference port 34. This results in that the first patch 2 is fedwith equal amplitude but with a phase difference of 180° at the oppositethird 20 and fourth 21 feeding points.

Also with reference to FIG. 2 c, which for reasons of clarity shows thepatches without the feeding arrangement, this in turn results in a thirdE-field 40 directed essentially perpendicular to the main surfaces 4, 5;6, 7 of the first 2 and second 3 patches in the circumferential slot 37created between the edges 8, 9 of the first 2 and second 3 patch,respectively, having a sinusoidal variation all around the circumferenceof the first 2 and second 3 patch. Using the same reference directionfor the fields, if the second E-field 39 varies with sine, the thirdE-field 40 varies with cosine. This means that the third E-field 40further is perpendicular to the second E-field 39, this will beexplained more in detail later.

In the same way as for the second mode of operation, the third E-field40 is shown in FIG. 2 c as a number of arrows having a length thatcorresponds to the strength of the E-field, where the arrows indicate aninstantaneous E-field distribution as it varies harmonically over time.

Thus, the triple-mode antenna arrangement 1 is now excited in threedifferent ways, thus acquiring three different modes with a first 38,second 39 and third 40 E-field, constituting aperture fields which allideally are orthogonal to each other.

The corresponding radiation patterns are also orthogonal, and thecorrelation equals zero, where the correlation ρ may be written as

$\rho = \frac{\oint_{\Omega}{{{\overset{\rightarrow}{E}}_{1}(\Omega)}{{\overset{\rightarrow}{E}}_{2}^{*}(\Omega)}{\Omega}}}{\sqrt{\oint_{\Omega}{{{{\overset{\rightarrow}{E}}_{1}(\Omega)}}^{2}{\Omega}{\oint_{\Omega}{{{{\overset{\rightarrow}{E}}_{2}(\Omega)}}^{2}{\Omega}}}}}}$

In the equation above, Ω represents a surface and the symbol * meansthat it is a complex conjugate. For the integration of the radiationpattern, Ω represents a closed surface comprising all space angels, andwhen this integration equals zero, there is no correlation between theradiation patterns, i.e. the radiation patterns are orthogonal to eachother. The denominator is an effect normalization term.

When determining that the radiation patterns are orthogonal, it ispossible to use the aperture fields. When considering the aperturefields, Ω represents an aperture surface. The aperture fields betweenthe edges 8, 9 are orthogonal since the integration of a constant (thefirst mode) times a sinusoidal variation (second or third mode) over oneperiod equals zero. Further, the integration of two orthogonalsinusoidal variations, sine*cosine, (the second and third mode) over oneperiod also equals zero. As these fields 38, 39, 40 are orthogonal atthe aperture of the antenna arrangement 1 and correspond to aperturecurrents (not shown) of the antenna 1, which aperture currents then alsoare orthogonal, the far-field also comprises orthogonal field vectors,as known to those skilled in the art.

Having three, at least essentially, orthogonal radiation patterns isdesirable, since this enables the rows in the channel matrix to beindependent. This in turn means that the present invention is applicablefor the MIMO system.

By means of superposition, all modes of operation may be operating atthe same time, thus allowing the triple-mode antenna arrangement totransmit three essentially orthogonal radiation patterns.

The actual implementation of the feeding arrangement is not important,but may vary in ways which are obvious for the skilled person. Theimportant feature of the present invention is that the patches 2, 3 arefed in three modes of operation, where the first mode of operationresults in an E-field 38 being acquired at the circumferential slot 38between the first 2 and second 3 patch. The other modes of operationresult in two E-fields 39, 40 which have sine variations of the fieldstrength being acquired at the circumferential slot 37 between the first2 and second 3 patch, where one of these E-fields is rotated 90° withrespect to the other. This function is not limited by the design of thefeeding arrangement or how the feeding points 18, 19, 20, 21 areconceived. They may for example obtain electrical connection in acontactless manner, i.e. by means of capacitive coupling as known in theart.

Due to reciprocity, for the transmitting properties of the triple-modeantenna arrangement 1 described, there are corresponding equal receivingproperties, as known to those skilled in the art, allowing thetriple-mode antenna arrangement to both send and receive in threeessentially uncorrelated modes of operation.

The invention is not limited to the embodiments described above, whichonly should be regarded as examples of the present invention, but mayvary freely within the scope of the appended claims.

Other types of patches may be conceivable, instead of those described.For example, the patches may have other shapes, for example square,rectangular or octagonal. The three patches may also have differentshapes between themselves, i.e. the first patch may be octagonal, thesecond patch square etc. The patches may be made in any appropriateconducting material, for example copper, aluminium, silver or gold. Thepatches may further be made from thin metal sheets and separated by aironly, held in place by means of appropriate retainers (not shown).Alternatively, the patches may be etched from copper-clad laminates.

Any kind of feeding of the patches is within the scope of the invention,where different kinds of probe feed are the most preferred. Thecapacitive probe feed mentioned above is such an alternative.

The distance d between the first imagined line and the respectivefeeding points does not have to be the same for every feeding point, butmay vary if appropriate. The positioning of the feeding points isdetermined by which impedance that is desired. In other words, thedistance d is generally varied in order to obtain a desired impedancematching.

The first imagined line does not have to pass through a central area ofthe patches, but may pass the patches wherever appropriate.

The feed network may further be implemented in many different ways,which ways are obvious for the person skilled in the art. The patchesmay be fed in such a way that other mutually orthogonal polarizationsmay be obtained, for example right-hand circular polarization and/orleft-hand circular polarization.

1-9. (canceled)
 10. An antenna arrangement comprising: first and secondplanar parallel antenna patches; and a feeding mechanism comprisingfirst, second, third, and fourth feeding lines for electrically feedingthe first antenna patch, wherein each feeding line is connected to thefirst antenna patch at feeding points angularly offset by 90 degreesfrom adjacent feeding points, wherein the first and second feedingpoints are offset by 180 degrees and the third and fourth feeding pointsare offset by 180 degrees such that the clockwise order of thesucceeding feeding points is the first, the third, the second, and thefourth; wherein when all of the feeding lines are fed essentially inphase, a first constant E-field is generated in a slot between the edgesof the first and second antenna patches; wherein when the first andsecond feeding lines are fed 180 degrees out of phase with each other, asecond, sinusoidally varying E-field is generated in the slot betweenthe edges of the first and second antenna patches; and wherein when thethird and fourth feeding lines are fed 180 degrees out of phase witheach other, a third, sinusoidally varying E-field is generated in theslot between the edges of the first and second antenna patches, saidthird sinusoidally varying E-field being rotated 90 degrees with respectto the second sinusoidally varying E-field.
 11. An antenna arrangementcomprising: a first and a second antenna patch, each patch constructedof a conducting material and having an upper and a lower main surface,said patches being placed one above the other with the first patch ontop such that all of the main surfaces are parallel to each other,wherein the first patch has a first edge and the second patch has asecond edge; a feeding arrangement comprising a first, second, third,and fourth feeding point feeding the first antenna patch, intransmission as well as in reception, wherein each feeding point ispositioned at a distance (d) from a reference point on the first patch,and the four feeding points are arranged around the reference point atapproximately 90-degree increments with the first and second feedingpoints being arranged on a straight line through the reference point andon opposite sides of the reference point, and the third and fourthfeeding points being arranged on a straight line through the referencepoint and on opposite sides of the reference point, wherein theclockwise order of the succeeding feeding points is the first, thethird, the second, and the fourth; wherein, in a first mode ofoperation, each one of the feeding points are fed essentially in phasewith each other, resulting in a first constant E-field being generatedin a slot formed between the edges of the first and second antennapatches; wherein, in a second mode of operation, the first and thesecond feeding points are fed 180 degrees out of phase with each other,resulting in a second E-field being generated in the slot, the secondE-field being directed between the edges of the first and second antennapatches and having a first sinusoidal variation along the slot; andwherein, in a third mode of operation, the third and the fourth feedingpoints are fed 180 degrees out of phase with each other, resulting in athird E-field being generated in the slot, the third E-field beingdirected between the edges of the first and second antenna patches andhaving a second, different, sinusoidal variation along the slot.
 12. Theantenna arrangement according to claim 11, wherein the arrangementoperates in the three modes of operation at the same time.
 13. Theantenna arrangement according to claim 11, wherein the first and secondfeeding points are fed with such phases, with respect to the third andfourth feeding points, that the second and third E-fields areessentially orthogonal to each other.
 14. The antenna arrangementaccording to claim 11, wherein the feeding arrangement also includes afirst and a second four-port, 90-degree, 3-dB hybrid junction and afirst and a second 90-degree phase-shifter; wherein the first hybridjunction comprises a difference terminal Δ₁, a sum terminal Σ₁ and twosignal terminals A₁ and B₁, and the second hybrid junction comprises adifference terminal Δ₂, a sum terminal Σ₂ and two signal terminals A₂and B₂; wherein the sum terminals Σ₁ and Σ₂ are connected to a commonsum signal at a sum connection point; and wherein: the signal terminalA1 is connected to the first feeding point through a first coaxial feedline, via the first 90-degree phase shifter; the signal terminal A2 isconnected to the third feeding point through a third coaxial feed line,via the second 90-degree phase shifter; the signal terminal B1 isconnected to the second feeding point through a second coaxial feedline; and the signal terminal B2 is connected to the fourth feedingpoint through a fourth coaxial feed line.
 15. The antenna arrangementaccording to claim 14, wherein all of the coaxial feed lines are ofequal length.
 16. The antenna arrangement according to claim 11, whereinthe antenna patches are symmetrical around the reference point.
 17. Theantenna arrangement according to claim 11, wherein the first and secondantenna patches have essentially the same shape.
 18. The antennaarrangement according to claim 16, wherein the first and second antennapatches are essentially circular.
 19. The antenna arrangement accordingto claim 11, wherein the distances (d) between the reference point andthe respective feeding points of the first antenna patch are essentiallyequal.